In a large nonstick skillet over medium heat, cook ground beef until browned, about 7-8 minutes, stirring to crumble. Drain.
Stir in ketchup and mustard, and continue cooking for about 3 minutes, stirring occasionally, until thoroughly combined and heated through.
Remove from heat and stir in onions, pickles, tomatoes, salt, and pepper.
Serve immediately on buns, topped with cheese.
Ketchup and pickles: We recommend purchasing natural or organic brands of these two items whenever possible, in order to avoid brands that have high-fructose corn syrup and other "questionable" ingredients.
Make-ahead and freezing tips: This reheats beautifully, so you can cook it entirely ahead and stash it in the fridge, then gently reheat it and assemble your sandwiches as needed throughout the week or for meal prep.
The base of this recipe can be frozen, to speed up dinner preparation on busy nights. If you'd like to use this as a make-ahead freezer meal, we recommend that you proceed only as far as cooking the ground beef and stirring in the mustard and ketchup, then cool and freeze the mixture WITHOUT ADDING the remaining ingredients until AFTER you've defrosted the ground beef mixture to finish and serve your sandwiches. The pickles, onions and tomato are meant to be fresh and lively in this recipe and won't hold up well through freezing.